The present invention relates to a method of manufacturing a screw washer including a mold for fabricating a screw washer and to a screw washer itself.
FIG. 4 illustrates a pipe clamp used for distributing oil-pressurized pipes or the like in a factory. A lower clamp member 105a is mounted on a lower washer 101, and in addition, an upper clamp member 105b is mounted on the lower clamp member 105a. A top plate 106 is disposed on the upper clamp member 105b by way of facing the lower washer 101. A bolt 107 penetrates the top plate 106 and the upper and lower clamp members 105b and 105a before being secured onto the lower washer 101.
As shown in FIG. 5, the lower washer 101 comprises a metal plate 102 and a positioning nut 103 which projects from the top surface of the metal plate 102. The lower clamp member 105a is positioned by internally coupling a positioning hole 105h of the lower clamp member 105a with the positioning nut 103. A tapped hole 104 corresponding to the bolt 107 is formed in the positioning nut 103.
The above-cited conventional pipe clamp is used for distributing pipe for feeding pressurized oil or air in a factory. However, in many cases, substantial mechanical vibration is transmissible in such a factory cited above, and therefore, it is essential that a substantial amount of torque be provided for fastening the bolt against the metal plate 102. In order to fully strengthen torque enough to fasten the bolt 107, this conventional pipe clamp uses the positioning nut 103 having length .beta. being greater than the thickness .alpha. of the metal plate 102. Concretely, the positioning nut 103 is formed independent of the metal plate 102. A through-hole 108 is formed in the metal plate 102 by way of penetrating the positioning nut 3, and then, the positioning nut 103 is inserted in the through-hole 108 before eventually brazing them.
In this way, since the conventional lower washer 101 comprises the discretely formed metal plate 102 and the nut 103 which are conjunctionally brazed, the manufacturing process involves a complexity which results in the difficulty to decrease cost. In addition, the brazing finish may be stripped off by an effect of vibration generated by a flow of fluid inside of the pipe clamp. A similar problem also occurs when using the conventional pipe clamp in an iron foundry in a highly-heated atmosphere.
Therefore, as shown in FIG. 6, there is such an idea to execute a method which initially forms a positioning nut 203 by punching out part of a metal plate 202 and then forms a tapped hole 204 in the center of the positioning nut 203.
More particularly, in order to provide the positioning nut 203 with a predetermined peripheral surface form, a die 300 having an aperture 301 having a shape corresponding to the positioning nut 203 is used. In addition, this method uses a trapezoidal projection 401 having a shorter length than the thickness of the metal plate in a range wider than the projection figure of the positioning nut 203. This method uses a punch 400 comprising a trapezoidal projection 401 projecting itself by way of being shallower than the thickness of metal plate in a range wider than the projection figure of the projection nut 203 and a center projection 402 which further protrudes from the center of the trapezoidal projection figure of the nut 203, and yet, the center projection 402 has a diameter wider than that of the tapped hole 204.
The trapezoidal projection 401 is wider than the aperture (recessed domain) 301 of the die 300 to expand the thickness of the positioning nut 203 which is formed by expanding a volume of the extruded part. Nevertheless, even when executing this method, in order to maximize a bonding strength between the positioning nut 203 and the metal plate 202, a certain volume is needed for the projection between the metal plate 202 and the positioning nut 203, and thus, a thickness of the positioning nut 203 cannot practically be expanded contrary to expectation, and thus, it results in the short length of the tapped hole 204 formed in the positioning nut 203.
For example, assume that a minimum of 4.8 mm of nominal height (as per JIS-B1181) of a nut available for a screw having 6 mm of nominal diameter "d", a minimum of 120 kgf/cm of bolt-fastening torque (as per JIS-B1052), and a minimum of 1.6 metric ton of peripheral tensile shearing force (as per JIS-B1051), are compulsorily demanded. In this case, even when forming the positioning nut 203 from a steel plate having 4.5 mm of thickness as per JIS and ISO standards to replace the positioning nut satisfying the above requirements if the requirements for the bolt-fastening torque and the peripheral tensile shearing force were fully satisfied, then, it conversely contracts the thickness of the positioning nut 203 below 4.8 mm. Conversely, if a minimum of 4.8 mm of thickness were provided for the positioning nut 203, then, it will cause the juncture .UPSILON. to become abnormally thin, thus failing to satisfy the above requirements prescribed for the bolt-fastening torque and the peripheral tensile shearing force.
On the other hand, there is another idea of directly forming a female screw on a 6 mm-thick steel plate conforming to JIS and ISO standards surpassing the JIS and ISO standards prescribing 4.5 mm of the steel-plate thickness. Nevertheless, in this case, substantially 5 mm of diameter is needed for the bottom hole. If the bottom hole having a narrower diameter than a thickness were punched out by means of a punch, then, it will incur an excessisve lead to the punch beyond tolerance. Instead, there is an idea of boring a bottom hole by applying a drilling machine. Nevertheless, this requires much operating time and processing work.